1. Field of the Invention
The present invention relates to circuit card or board and, in particular, to its internal plane structure.
2. Prior Art
The electromagnetic interference (EMI) radiated by electronic products is controlled by regulatory bodies both in the U.S. and in foreign countries, who set emission standards for electronic products. If a product does not meet the applicable emission standards, the manufacturer may be fined exorbitant fees and, in extreme situations, the product may have to be withdrawn from the market.
To avoid these inconveniences, most manufacturers have built EMI facilities for testing products prior to marketing the products. The facilities are expensive and require personnel trained in the EMI discipline to operate them. Even after the initial release of a product, there is a need to recertify the product if it is to be used in a different system or if a new feature is added. The certification and recertification of products lead to an ever-increasing workload which demands more technicians and testing facilities. The net result is that the certification recertification of products has become costly and will become more so in the future.
It is believed that a major cause of radiated EMI is common-mode current on system cables. The common-mode current is caused by thousands of digital circuits that are on the circuit board or card, many of which are switching simultaneously. As the circuits switch, large current surges are generated causing high-Q, parallel-resonant circuits between the card's voltage and ground planes.
One prior art approach for solving the common-mode current problem is to decouple the digital logic. This is done by placing a series inductor and bypass capacitor or bypass capacitor alone between a module's power pin and the Vcc/ground planes. This solution is a step in the right direction in that it suppresses the problem at the source. For this approach to be completely effective, a decoupling network would be required for each active device on the card/bard. Due to the large number of active devices, providing separate decoupling networks for each device would be impractical and costly. Instead, what is done is to try and place a decoupling network on a few devices that are believed to be generating the most noise and only use bypass capacitors elsewhere. As a result, this technique is usually inadequate and requires additional techniques for solving the noise problem.
The ground isolation method is another technique used in the prior art. In this technique, a large ground sink (metal or metalized cabinet) is coupled via a very low impedance connection to the isolated ground plane. This implies that all card cable connectors must be located in the same area as the isolated ground and have the capability of being strapped to the metal sink. This technique is unsatisfactory since it is not always possible to have the cables in the same location.
Another method used to solve the prior art problem is the placing of large ferrite cores on the offending cables. The effectiveness of this approach is limited in that its attenuation is usually less than 4 to 6 dB. Moreover, the FCC prohibits this approach unless the seller provides all the cables with the products. This is not usually practical since a customer may elect to provide their own cable.
Finally, the transformer/balun approach is another technique used in the prior art. In this technique, a balun/transformer is mounted on the card. This approach is not practical in that its effectiveness is typically less than 4 dB and requires that all wires in an offending cable pass through the device. Furthermore, one input pin and one output pin, with traces, are required for each wire. All of this is difficult to accomplish with large cables and is usually limited to cables with 8 or 10 wires. In addition, unless all system cables are suppressed in the same manner, the noise energy tends to bypass the suppressed cable. Moreover, when any cable length is changed, the effectiveness of this approach is changed over the entire frequency spectrum and the system usually exhibits different failing frequencies.